103 



but the proximal articular cavity is subcircular, as in the Megatherium 

 and Megalonyx. 



The distal half of the outer margin of the radius is convex ; the bici- 

 pital tuberosity is nearer the proximal end and nearer the inner margin 

 of the bone ; the posterior surface of the radius more resembles that in 

 the Megalonyx, Like the perforated humerus, the present bone in the 

 Scelidothere also exhibits modifications which connect the Mylodon with 

 the Megalonyx. 



500. The right femur, wanting the distal extremity ; its fractured end demon- 



strates the absence of the medullary cavity. 



501. The left femur. This is a very remarkable bone, both on account of its 

 great proportional size as compared with the body, and its extreme 

 breadth as compared with its length or thickness ; but in all these circum- 

 stances the affinity of the Scelidothere with the Megathere is prominently 

 brought into view. The breadth of the femur, though considerable, is 

 less marked in the Mylodon ; and there are no other known quadrupeds 

 with which the Scelidothere can be compared in this respect. In pro- 

 ceeding, however, to compare together the thigh-bones of the Scelido- 

 there and Megathere, several differences present themselves which are 

 worthy of notice : of these the first is the presence in the Scelidothere of 

 a depression for a ligamentum teres on the back part of the head of the 

 femur, near its junction with the neck of the bone. 



The head itself forms a pretty regular hemisphere : the great tro- 

 chanter does not rise so high as in the Megathere, but relatively it equals 

 it in its breadth. The small trochanter is proportionally more deve- 

 loped ; the external contour of the shaft of the femur is straighter in the 

 Scelidothere than in the Megathere, and the shaft itself is less bowed 

 forwards at that part. The articular condyles occupy a relatively smaller 

 space upon the distal extremity of the femur in the Scelidothere, and 

 they differ more strikingly from those of the Megathere in being con- 

 tinued one into the other : the rotator surface, for example, is formed 

 by both condyles, while in the Megatherium it is a continuation exclu- 

 sively of the external articular surface on the outer condyle. 



